Method of forming moulds for precision casting



Mnrnonor roar/ruse MoULns'roR, a PnEcrs oNcAsTiNG HaroldGart'on E mhlem, Melhourne, and Desmond Joseph Cloherty, Chellaston, England, assignors to Rolls- Royce Limited, Derby, England No Drawing. Application July 19, 1954 I I gSerialNo. 444,364

Claiins priprity, application Great Britain Augu'st18; 1953 9 Claims; (Cl. 106-383) In precision investment casting'processes a pattern is invested in a refractory material and then subsequently removed producing a mould in which the metal is cast to obtain a cast article. 1

The pattern is commonly'made from wax or otherlow melting expendable material, and is then removed by melting, dissolving in' a suitable solvent-or by some 1 mechanical process Mouldsrmay' also be formed by investing separately Idivided' patterns ofl a solid non-expendable nature which aresubsequently removed leaving a number of part moulds'which can be assembled together to forr'n a com Pleteimould. Such part moulds are for-convenience included. in the term mould used'herein.

Y 'T The investment material may also be used for the pro duction of cores which are assembled in the casting mould, and in speaking hereinafter of moulds it is to be understood that'cores are also contemplated.

I It is known-to prepare -a;bi-nding agent forthe refractory material by-carefully-reacting an aqueous-solution of tin-inorganic alkali metal silicate, generally sodium silicate,- with anaqueous "solution"of a-mineral acid, generally-either nitric aeid'orhydrochloric acid.

1'1ypical binders prepared by this process-contain about vSpercentof silica and have a shelf life of about'24 hours.

1' One disadvantage of this binder istha-t thealka'li metal associated with the soluble silicate acts to reduce the refractori'ness of the mould, which in tum-may, give rise to the formation of slag when casting takes 'place'.

Binders 0f higher'silica content would be expected to give stronger moulds, thus enabling more intricately shapedobjects to be cast. If'however, the proportion of alkali metal silicate were to beincreased; a still greater proportion. ofalkali metal would be present, reducing still- 7 furthertherefractoriness of the mould and at -the same time. the shelf life of the liquid would be shorten We have'now found that a mould having greatly increased; strength may be prepared'from a refractory sion'ofsilica in water with the acid reaction "products formed byadding an aqueous.solution-ofanalkali metal silicate to .an excess of an aqueous solution of a mineral, acid;

[A "stable substantially electrolyte-free; colloidal disper sion of silica in water is frequently silicasol or aquasol.

: By means of thisii vention the concentration of silica 1 inthe ,liquidrisr increased. without. at the sameitime j increasingthe alkali metal concentration v.andthere is thus produceda mouldwhich is both stronger andi'more refractory..

. f The strength of: the mouldcan be varied byf ch'anging "the amount of silica sol which is added. 1

---'The excess-of aqueous solution of--.mineral'-acid to which the aqueous solution of alkali metal silicateis ad'dedshould preferably not be so great-as to make the referred to as a stable acidzreaction products formed thereby more than 0.08- N V It is, however, essential to have 'su-fiicient acid-present V to react with any alkali. present in'the refractory filler] and leaverthe investment. mixture slightly acid because if sufficient acid is not present the investment mixture will set very rapidly. This is' particularlyimportantif the, refractory. filler contains an appreciable proportion of magnesia. p v} If therefore, more acid'is required than is present in the: acid reaction products'as before stated, extraacid should beadded to these' products immediately before the addition of the? silica-:sol during: which operation the mixtureshouldbestirred? v 5 Examples of suitable. soluble silicates for-'the'production of the acid reaction products are sodium silicate,

' potassium silicate-and ammonium silicate A. preferred sodiunr'silicatehas a specific gravity -of" 113' and: a

Na ofsio ratio of- 1:3.8'. J

Examples 5 of suitable mineral acids for the production of the acid reaction productsare hydrochloric acid, phosphoric acid and nitric acid. Of these, nitric acid preferred. r

' filhe' silica'=.sol:inay beprepared by reacting a water soluble inorganic. silicate suchas'sodiu-m silicate with an ion exchange resin inknown manner, and concentrats ing the resultantsolution-to:a silica concentration of at least 30 percent. v

Other methods of pr'epaiingsuch silica sols are" known and may-b'e employed. i x Typicalsilica sols' which may be used in this invention are sold under the'rcgiste'red trademark may be prepared include silica, the aluminum silicates such asmullite,sillimanite,calcined kaolin and calcined fireclay grog, together with Zircon and zirconia. In these materials the lime, magnesia and alkali content prefermaterialby'usinga binding liquid comprising a mixture ofia stable substantiallyelectrolyte-free colloidal disperagents and/or foam inhibitors.

ably shouldjnot exceed 4%. It will be realised tha-t other refractory. materials may be used in addition to. those listed, providedthey do not have an excessive alkali content Tli'e binder liquid may also include wetting On'e ekample of the p epara tion of an investment mould using a binder liquid according to this invention is as follows: i i i I 7 The binder is prepared as follows:

4,000 cc. of sodium silicate solution having a s pecific gravity 1.3 and a NagO/SiOg ratio of l: 3.8 are poured into 6,000 cc. of water and, thoroughly stir'red. I ;'1 he resulting solution'is adjusted byjthe' addition, of water or sodium silicatefuntil 25 cc; of solution are equivalent to .28'.0i0.1 cc. of' N/ l hydrochloric acid ,using screened methyl orange? asin'dicatorI- .Theisodium silicate solution a ,isthus'between 1.124 N and; 11116 N in. alkalii v 1,000. cc. of concentrated nitric acid are diluted with 10,000 cc; of Waterand thoroughly stirred. The solu-' tion adjusted by dilution with water or'addition of nitric acid' until 25 c'clofsolution are equivalent j to Patented July 8,195

carrying out 30 Vpercent.

Suitable refractory materials, from which'the mould centrated hydrochloric acid solution.

380:0.1 cc. of N/1 sodium hydroxide using screened methyl orange as indicator. The nitric acid solution is thus between 1.524 N and 1.516 N in acid.

5,460 cc. of the dilute sodium silicate solution are run into 4540 ccs. of the rapidly stirred dilute nitric acid solution. During this operation 'it is essential to prevent too rapid addition of the sodium silicate solution in order to avoid gelation while mixing. When mixing is complete the solution is adjusted either by the addition of dilute sodium silicate or by addition of dilute nitric acid until 100 cc. of solution are equivalent to 8.0:01 cc. of N/ 1 sodium hydroxide using screened methyl orange as indicator. After adjustment of the solution, 16 cc. of 2 ethyl hexanol were added as'a foam inhibitor together with 42 cc. of Teepol wetting agent. Teepol is the registered trademark under which is sold a surface active agent based on sodium higher alkyl sulphates.

The mould investment slurry is then prepared as follows:

6,300 cc. of the binder liquid prepared as above were further acidified with 330 cc. of concentrated hydrochloric acid solution. 15,800 cc. of Syton 2X (silica sol No. 2) were added-immediately, the liquid being stirred during the addition. A mould investment slurry wasfprepared by adding to the resulting liquid 224 lbs. of a refractory filler containing approximately equal parts by weight of fine zircon sand and calcined kaolin. The refractory filler had the chemical analysis and particle size distribution given below. The addition of concentrated hydrochloric acid is necessary to counteract the alkalinity of the filler. 1

On vibration of the invested patterns to form a mould by consolidating the refractory in the slurry a considerable volume of surplus liquid is obtained when packing is complete. This surplus liquid may be recoveredfor further use after the addition of acid to inhibit gelation.

Analysis of refractory filler Percent 3150 cc. of the acid reaction products formed by adding an aqueous solution of sodium silicate to an aqueous solution. of nitric acid,.prepared as described in Example 1, were further acidified with 120 cc. of con- 10,000. cc. of Syton 2X (silica sol No. 2) were added immediately, the liquid being stirred during the addition. A mould investment slurry was prepared by adding to the resulting liquid 112 lbs. of a refractory filler containing approximately equal parts by weight of fine zircon sand and calcined kaolin. The refractory filler had the same chemical analysis and particle size distribution as the filler used in Example 1.

These quantities give a much more fluid investment slurry, which therefore packs in less time when the invested pattern is vibrated and consequently speeds up the investment process. The gel time of the investment slurry may be lengthened by increasing the amount of concentrated hydrochloric acid solution. An amount in excess of cc. gives an in-conveniently long gel time. A considerable volume of surplus liquid is obtained when packing is complete. This surplus liquid may be recovered for further use after addition of acid to inhibit gelation.

If desired, the Syton 2X (silica sol No. 2) may be replaced by the same volume of Syton C (silica sol No. 1). However, this gives a weaker mould.

EXAMPLE 3 3360 cc. of the acid reaction products formed by adding an aqueous solution of sodium silicate to an aqueous solution of nitric acid, prepared as described in Example 1, were further acidified with 250 cc. of concentrated hydrochloric acid solution. 15,000 cc. of Syton 2X (silica sol; No. 2) were added immediately, the liquid being stirred during the addition. 112 lb. of calcined high alumina fireclay grog were added to the resulting liquid, to give a mould investment slurry from which the mould was formed by vibration of the invested pattern to consolidate the refractory in the slurry. The calcined fireclay grog had an alumina content of 44% the particle siz distribution being given below.

B. S. S. 410 sieve No.: Percent retained The increase in the volume of concentrated hydrochloric acid is necessary because of the relatively high alkali content of the fireclay grog.

EXAMPLE 4 6,000 cc. of the acid reaction products formed by adding an aqueous solution of sodium silicate to an aqueous solution of nitric acid, prepared as described in Example 1, were further acidified with 250 cc. of concentrated hydrochloric acid, being approximately 5% of the products volume. 12,000 cc. of Syton 2X (silica sol No. 2) were added immediately, the liquid being stirred during the addition. A mould investment slurry was prepared by adding to the resulting liquid 26 lbs. of zircon, all of which passed a 200 mesh B. S. S. 410 sieve, followed by 1 cwt. of calcined high alumina fireclay grog, which had an alumina content of 44%, the particle size distribution being given below.

B. S. S. 410 sieve No.: Percent retained The mould was formed by vibration of the invested pattern to consolidate the refractory in the slurry.

' preferably for not less than ten 1 sol.

. EXAMELE 5 10 litres of liquid recovered from moulds prepared from the investment slurry described in Example 2 using Syton 2X (silica sol. No. 2) wereacidified with 175 cc.

of concentrated hydrochloric acid.- 3150 cc. of Syton 2X (silica sol No. 2) i were then added, the mixture being stirred during this addition; A mould investment slurry was prepared by adding to the resulting mixture .1 12 lbs. of therefractory described in Example 1. The mould was formed by vibration of the invested pattern to consolidate the refractory in the slurry. p

' In whatever way the slurryis prepared, the resulting moulds after vibration are left .at normal temperature for 24,to 48 hours, to gel the binding liquid. The mould is then heated at a temperature between 95 and 100 C. for atleast 24 hours, to remove the wax pattern if such a pattern has been invested.

be'no such pattern in the case of a core.) The mouldv is then gradually heated to a temperature of approxi- (There will of course mately 1,000 C. over a period of 8'hours,-andvis held at approximately 1,000 C. for at least two hours, and hours prior to the casting operation.

By means of this invention it has been found p'os-' sible to prepare highly refractory moulds of great strength. These moulds have been found to give satisfactory results when the article'to be cast incorporates one for more longslender cores as are used in the production a flof certain hollow castings, for example hollow turbine stationary blades of gas turbine engines.

A further advantage of the binding liquid according to this invention is'thatit is water based and therefore does not give rise to a fire or explosion risk.

What we claim is: v 1. The'meth-od of forming a mould for precision casting which includes adding an aqueous solution of an alkali metal silicate to'an excess of an aqueous solution of a mineral acid, mixing with the resultant acid reaction products so formed a stable substantially electrolyte free colloidal dispersion of silica in water alone and using the resultant liquid to bind a refractory mould material.

2. The method claimed in claim lin which the acid reaction products are not more than 0.08 N in acid;

3. The method claimed in claim 1 in which extra acid is added immediately before the addition of the silica 4. The method "claimed in claim 1 in'which the silicate is selected from the group consisting of sodium silicate, potassium silicate and ammonium silicate.

5. Themethod claimed in claim 1 in which the acid used for the production of the acid reaction products is selected from the group consisting 1 phoric and nitric acids.

of hydrochloric phos- 6. The method of preparing a mould for precision casting which includes forming a slurry by adding an aqueous solution of an alkali metal silicate to an excess of an aqueous solutionof a mineral acid, mixing with the acid reaction products so formed a stable substantiallyv electrolyte-free colloidal dispersion of silica inwater and adding a refractory material to the binding liquid'so 7 formed, investing anxexpendable pattern with said slurry, allowing the mould to stand while the binding liquid ,7

' gels, causing removal of the expendable pattern by gradthe acid reaction products so formed a stable substanually heating the mould to a temperature of the order of 1000 C. and holding the same at such temperature for e V at least 2 hours before casting.

7. The method ofpreparin'g a core element forpre'cision casting which includes forming a slurry byadding an aqueous solution of an alkali metal silicate to an excess of an aqueous solution of a'mineral acid, mixing with tially electrolyte free colloidal dispersion of silica in Water and adding a refractory material to the binding'liquid so formed, shaping the slurry to the required core form, allowing the same to stand while the binding liquid gels, gradually hea'ting'the same to a temperature of the order of 1.000 C. and holding the same at such a temperature for at least 2 hours before casting.

reaction products of a stable substantially electrolyte'free colloidal dispersion of silica in water only,;andusing the resultant liquid to bind a refractory mould material.

9. The method of forming a mould for precisioncasting which includes adding .a slightly alkaline reaction aqueous solution of analkali metal silicate to such a quantity of an aqueous solution of a mineral acid as to' I produce acid reaction products of not more than 0.08 N

acid, adding not more than about 5% of the products volume of a concentrated mineral acid with stirring, im-' mediately'mixing with the acidified acid reaction products aquantity of a stable substantially electrolyte free colloidal dispersion of silica in water only,.in an'ameunt of approximately twice that of the said products, and using the resultant liquid to bind a refractorymould material.

References Cited in the fileof this patent UNITED STATES PATENTS 2,380,945 Collins Aug. 7, 2,441,695 Feagin et al. 'May.18, 1948 2,524,358 Robey Oct. 3, 1950 2,601,235 Alexander et al June 24, 1952 2,680,890 Moor et al. June 15, 1954 2,701,902 Strachan ,Feb. 15, 1955 OTHER REFERENCES Iler: published by Cornell University Press (1955), pages 45, 48-49. v

The Colloid Chemistry of Silica and Silicatesfl 

1. THE METHOD OF FORMING A MOULD FOR PRECISION CASTING WHICH INCLUDES ADDING AN AQUEOUS SOLUTION OF AN ALKALI METAL SILICATE TO AN EXCESS OF AN AQUEOUS SOLUTION OF A MINERAL ACID, MIXING WITH THE RESULTANT ACID REACTION PRODUCTS SO FORMED A STABLE SUBSTANTIALLY ELECTROLYTE FREE COLLOIDAL DISPERSION OF SILICA IN WATER ALONG AND USING THE RESULTANT LIQUID TO BIND A REFRACTORY MOULD MATERIAL. 